The main electrical parameters such as ideality factor (n), zero bias barrier height (square(Bo)), series resistances (R(s)), depletion layer width (VD) and interface state densities (N(ss)) of (Ni/Au)/AlGaN/AlN/GaN heterostructures have been extracted from the current-voltage (I-V) at room temperature, and frequency dependent capacitance voltage (C-V) and conductance-voltage (G/w-V) measurements. The high value of n and Rs were attributed to the existence of an interfacial layer (IL) and particular distribution of Nss. The density distrubition profile of N(ss) was obtained from both forward bias I-V data and low-high frequency (C(LF)-C(HF)) measurement methods. In addition, the voltage dependent R(s) profile obtained both I-V and admittance measurements are in good agreement. As a result, the existence of an IL, R(s) and N(ss) lead to deviation from the ideal case of these heterostructures.